Direct engraving of a flexography plate requires carving three dimensional (3-D), on plate material, directly with a laser system. This is remarkably different from two dimensional (2-D) imaging techniques that require post processing steps to produce the 3-D features.
This difference introduces several challenges for the laser imaging system:
                1. The laser system must have sufficient power to ablate the material; and        2. The laser spot should be small enough to achieve the fine detail required in quality printing.Although high power density does not necessary conflict with laser focusability, from a practical perspective, these lasers offer significantly higher cost per watt of output optical power than broad spot lasers. As a result, it is desirable to operate with broad laser sources, that may produce high output optical power, rather than with small spot sources, that may have high power density but relatively low total power output.        
It is therefore appealing to use a laser system that combines the characteristics of a fine spot laser source to process areas which require fine detail screening and a broad spot laser source for portions of the image where features comprise large solid areas.
U.S. Pat. No. 6,857,365 to Juffinger et al provides a method of producing a printing block by introducing a relief is into a surface of a printing block blank. To form the relief, material of the printing block blank is removed along tracks by radiation. The relief regions may be formed at different depths along one and the same track by frequent exposure to radiation by radiation sources mounted on the same optical head.
U.S. Published application No. 2006/0065147 to Ogawa provides a method of engraving a flexo direct printing plate in two processes. One is a precision engraving process for irradiating the flexo direct printing plate at a precision engraving pixel pitch with a precision engraving beam having a small diameter, to engrave the plate to a maximum depth. The other is a coarse engraving process for irradiating the flexo direct printing plate at a coarse engraving pixel pitch larger than the precision engraving pixel pitch, with a coarse engraving beam having a large diameter, to engrave the plate to a relief depth. A variable beam expander changes the diameter of the laser beam emitted from the single laser source.
U.S. Pat. No. 6,150,629 to Sievers provides a laser engraving machine used for engraving a workpiece surface by a modulated laser beam in order to form a desired profile in the workpiece surface. The fine structures of the profile are formed by the laser beam of a first laser which is modulated by an acoustooptic modulator with relatively high modulation frequency, while the deep areas of the desired profile are formed by the laser beam of a second laser, for which purpose the modulator, on the one hand, and the second laser beam source, on the other hand, are driven by interrelated but separate control signals. The two perpendicular polarized laser beams from the modulator and the second laser beam source are transmitted and reflected by a selective mirror, respectively, and applied commonly via a single optical system to the workpiece surface to be machined.
U.S. Published patent application No. 20060132592 to Sievers provides for the transferring of an image by the combined flux from two or more beams of light. Particular embodiments ablate the mask printing plates for CTP systems by the combined illumination from a first, broad beam and a plurality of controllable, pulsed beams that co-illuminate the plate with the first beam.
U.S. Published patent application No. 20060203861 to Ogawa provides a laser engraving machine having a recording drum rotatable with a flexo sensitive material mounted peripherally thereof, and a recording head movable parallel to the axis of this recording drum. The recording head includes a first laser source for emitting a precision engraving beam L1, a second laser source for emitting a coarse engraving beam L2, an AOM for modulating the precision engraving beam L1, an AOD for causing the precision engraving beam L1 to scan axially of the recording drum, an AOM for modulating the coarse engraving beam L2, a synthesizing device, and an optic for condensing the precision engraving beam L1 and coarse engraving beam L2 synthesized by the synthesizing device on the flexo sensitive material.